BMW 530I COOLANT PROBLEM

2003 BMW 530i • 87,000 miles

Recently replaced thermostat in 2003 530i three days ago. Everything seemed to be going well until today when I noticed 'condensation' on the inside of the car (the car was parked and not running). I started the vehicle, turned on the heat and the condensation disappeared. The vehicle has been turned off for about 2 hrs and the condensation has returned. The temperature outside is about 37 degrees. What's going on?